The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2001-328765 filed on Oct. 26, 2002, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an ink jet recording head and an ink jet recording apparatus in which ink droplets are ejected from nozzles so as to record characters or graphics.
2. Description of the Related Art
Drop-on-demand type ink jet systems are generally known well (JP-A-Hei.53-12138 and JP-A-Hei.10-193587). In the drop-on-demand type ink jet systems, a pressure generating member such as a piezoelectric actuator is used to generate a pressure wave (acoustic wave) in a pressure generating chamber filled with ink, so that an ink droplet is ejected from a nozzle communicating with the pressure generating chamber by the pressure wave.
FIG. 23 shows an example of an ejector in an ink jet recording apparatus known in these official gazettes. An ejector 10 is constituted by a common flow path 4, an air damper 26, an ink supply path 5, a diaphragm plate 8, a piezoelectric actuator 3, a pressure generating chamber 1 and a nozzle 2. Generally, one ejector 10 has one nozzle 2. Generally, one ejector 10 has one nozzle 2. The nozzle 2 for ejecting ink and the ink supply path 5 for introducing ink from an ink tank (not shown) through the common flow path 4 communicate with the pressure generating chamber 1. The air damper 26 is provided on the common flow path 4 so as to absorb the pressure. In addition, the diaphragm plate 6 is provided on the bottom surface of the pressure generating chamber 1, and the piezoelectric actuator 7 is attached to the outside of the diaphragm plate 6.
To eject an ink droplet 8, the piezoelectric actuator 7 displaces the diaphragm plate 6 to change the volume of the pressure generating chamber 1 to thereby generate a pressure wave. By this pressure wave, a part of ink charged into the pressure generating chamber 1 is jetted to the outside through the nozzle 2 so as to fly as the ink droplet 8. The flying ink droplet 8 lands on a recording medium such as a recording paper so as to form a recording dot. Such a recording dot is formed repeatedly in accordance with image data. Thus, characters or graphics are recorded on the recording medium.
FIG. 24 schematically show the meniscus operation of the nozzle 2 before and after ejecting the ink droplet 8. As shown in FIG. 24A, a meniscus 9 is substantially flat initially. When the pressure generating chamber 1 is compressed, the meniscus 9 moves toward the outside of the nozzle 2 so as to eject the ink droplet 8 (FIG. 24B). Immediately after the ink droplet 8 is ejected, the ink volume inside the nozzle 2 is reduced so that the meniscus 9 is formed into a concave shape (FIG. 24C). The value y shown in FIG. 24C designates the displacement of the meniscus 9 after the ejection. The concave meniscus 9 undergoes the states shown in FIGS. 24D and 24E by the effect of the surface tension of the ink, and returns to the opening portion of the nozzle 2 gradually. Thus, in a short time, the meniscus 9 recovers its condition before the ejection (FIG. 24F).
FIG. 25 shows the positional change of the meniscus 9 immediately after the ejection of the ink droplet 8. The meniscus 9 making a large retreat (y=xe2x88x9260 xcexcm) immediately after the ejection (t=0) returns to its initial position (y=0) while swinging as shown in FIG. 25. The return behavior of the meniscus 9 after the ejection of the ink droplet 8 is referred to as xe2x80x9crefillxe2x80x9d, and time for the meniscus 9 to return to the opening surface of the nozzle 2 for the first time after the ejection of the ink droplet 8 is referred to as xe2x80x9crefill timexe2x80x9d (tr).
In ink jet recording heads, the number of nozzles 2 is a parameter having the greatest influence on the recording speed. As the number of the nozzles 2 is increased, the number of dots that can be formed per unit time is increased so that the recording speed can be enhanced. Therefore, in a typical ink jet recording apparatus, a multi-nozzle type recording head in which a plurality of ejectors 10 have been interconnected one another is often adopted. FIG. 26 shows a recording head in which ejectors 10 are aligned one-dimensionally. The recording head is constituted by an ink tank 20, ink conduits 18a and 18b, a filter 19 and the ejectors 10. The ink tank 20 is connected to a common flow path 4 through the ink conduits 18a and 18b and the filter 19. The plurality of ejectors 10 communicate with the common flow path 4.
However, in such a structure in which the ejectors 10 are aligned one-dimensionally, the number of the ejectors 10 cannot be increased much. It is said that the upper limit of the number of ejectors 10 is typically about 100. Therefore, some ink jet recording heads in which the number of ejectors is increased by arraying ejectors two-dimensionally in a matrix (hereinafter, referred to as xe2x80x9cmatrix-array headxe2x80x9d) have been heretofore proposed (JP-A-Hei.1-208146, JP-A-Hei.10-508808, etc.).
FIG. 27 shows an example of a matrix-array head. The matrix-array head differs from the recording head in FIG. 26 in that a second common flow path 16 is provided newly and there are a plurality of common flow paths 4. Each of common flow paths 4 communicate with the second common flow path 16, and a plurality of ejectors 10 are connected to each of the common flow paths 4. Such a matrix-array head structure is very effective in increasing the number of ejectors 10. For example, when the number of common flow paths 4 is set at 26 and 10 ejectors 10 are connected to each of the common flow paths 4, 260 ejectors 10 can be arrayed.
FIG. 28 shows an ink jet recording head disclosed in JP-A-Hei.10-508808. FIG. 28A shows the section of ejectors 10, and FIG. 28B shows the schematic arrangement of the ejectors 10. As shown in FIG. 28A, the ink jet recording head is constituted by pressure generating chambers 1, nozzles 2, communication paths 3, ink supply paths 5, a diaphragm plate 6, piezoelectric actuators 7 and flow paths 23. This ink jet recording head is formed by laminating a nozzle plate 11, a flow path plate 25 and the diaphragm plate 6 to one another. Partition walls 27 are thick enough not to transmit the pressure in the pressure generating chambers 1 to the flow paths 23. As shown in FIG. 28B, the flow paths 23 communicate with the flow paths 24. The flow paths 23 correspond to the common flow paths 4 in FIG. 27, and the flow paths 24 correspond to the second common flow path 16.
In the related-art matrix-array heads as shown in FIGS. 27 and 28, however, there are some problems. As for the first problem, the interval (nozzle pitch Pc) of nozzles 2 having a common flow path 4 therebetween cannot be set to be small. As a result, the array density of the ejectors 10 (the number of nozzles per unit area) cannot be made very high.
FIG. 29 shows an equivalent electric circuit of the matrix-array head. The signs m, r, c, and "PHgr" designate inertance [kg/m4], acoustic resistance [Ns/m5], acoustic capacitance [m5/N] and pressure [Pa], respectively, and suffixes d, c, i, n, p and pxe2x80x2 designate a driving portion, a pressure generating chamber, an ink supply path, a nozzle, a common flow path and a second common flow path, respectively. In the matrix-array head having the ejectors 10 arrayed two-dimensionally, as shown in FIG. 29, a large number of ejectors 10 communicate with one another through the common flow paths 4 and the second common flow path 16. Therefore, when the number of ejectors 10 communicating with one and the same common flow path 4 is large, it is necessary to suppress crosstalk (pressure interference) or the like between the ejectors 10 close to each other. It is, therefore, necessary to secure a large acoustic capacitance in the common flow path 4.
However, as will be described later, in order to increase the acoustic capacitance of the common flow path 4, it is necessary to set the width of the common flow path 4 to be large. Accordingly, in the related-art matrix-array heads, the nozzle pitch Pc between the nozzles 2 opposed to each other through the common flow path 4 becomes so large that the nozzles 2 cannot be arrayed with high density.
Further, as the second problem in the related-art matrix-array heads, the ejection condition becomes unstable when large-diameter ink droplets 8 are ejected concurrently from a plurality of ejectors 10 in a short period (high-frequency concurrent ejection) from a plurality of ejectors 10. FIG. 30 shows an example of results of testing about the stability of ejection using a related-art matrix-array head, in which the droplet volume of each ink droplet 8 and the ejecting frequency thereof were varied. A designates the result when the ink droplet volume was 20 pl, and B designates the result when the ink droplet volume was 30 pl. Incidentally, the stability of ejection was evaluated as a change of the flying speed (droplet speed) of the ink droplet 8.
As shown in the graph A, when ink droplets each having a droplet volume of 20 pl were ejected concurrently from 260 ejectors 10 arrayed in a matrix, it was confirmed that the droplet speed became unstable at the ejecting frequency of 10 kHz or higher, and the droplets could not be ejected at the ejecting frequency of 15 kHz or higher. The ejecting condition of ink droplets 8 at that time was observed stroboscopically. As a result, the ejecting condition that large-diameter droplets and small-diameter droplets were ejected alternately was often observed at the ejecting frequency of 10 kHz or higher, and the case where the droplet diameter or the droplet speed changed at random was also observed. In addition, when the droplet volume was increased to 30 pl, similar unstable ejection was observed at the ejecting frequency of 7 kHz or higher as shown in the graph B.
As a result of the experimental evaluation, it was found that the phenomenon that the ejection became unstable was apt to occur when the number of ejectors 10 serving for concurrent ejection was large, when the ejecting frequency was high, or when the diameter of ink droplets 8 to be ejected was large. In addition, it was confirmed that when unstable ejection occurred, all the ejectors 10 connected to the same common flow path 4 became unstable in the substantially same manner. From the result of such observation, it can be said that the phenomenon of unstable ejection is not caused by acoustic crosstalk among the ejectors 10 but is a new phenomenon of unstable ejection that has never been brought into question in the related art.
When such an unstable ejection phenomenon occurs, the droplet volume or the droplet speed of the ink droplets 8 becomes very unstable so that the quality of an output image is degraded on a large scale. In addition, when the degree of instability is conspicuous, bubbles may be involved in the nozzles 2 so as to inhibit ejection. Since such an unstable ejection phenomenon occurs, the related-art matrix-array heads cannot eject large-diameter ink droplets 8 concurrently from a large number of ejectors 10 at a high frequency. Thus, the related-art matrix-array heads cannot sufficiently exert their feature that the matrix-array heads are advantageous to high speed recording.
It is an object of the present invention to provide a matrix-array head which can be produced with a high nozzle array density and at low manufacturing cost, and further in which an unstable ejection phenomenon occurring when large-diameter ink droplets are ejected concurrently from a plurality of ejectors at a high frequency is suppressed so that stable, high-speed recording can be carried out.
The present inventor made various researches in order to prevent crosstalk from occurring among ejectors in ink jet recording heads. As a result of the researches, the inventor obtained the following finding and achieved the invention. Here, there is a close relationship between the acoustic capacitance of a common flow path 4 and the width of the common flow path 4. When the rigidity of the common flow path wall is high, the acoustic capacitance cv of the common flow path 4 is expressed by the following equation.
cv=Vp/(xcexaxc2x7Kr)xe2x80x83xe2x80x83(1)
where Vp designates the volume [m3] of the common flow path 4, xcexa designates the elastic coefficient [Pa] of ink, and Kr designates a correction coefficient depending on the rigidity of the common flow path wall, which is typically a value of about 0.3-0.7. The acoustic capacitance of the common flow path 4 is proportional to the volume Vp of the common flow path. Since there is an upper limit on the height of the common flow path 4 (typically about 100-200 xcexcm), the width of the common flow path 4 has to be set to be large enough to secure a large acoustic capacitance (volume).
In addition, when an air damper 26 having a small rigidity is added to a part of the common flow path 4 as shown in FIG. 23, the acoustic capacitance of the common flow path 4 may be also increased. In this case, the acoustic capacitance cd added to the common flow path 4 by the air damper 26 can be calculated by:                               c          d                =                                            l              d                        ⁢                                          W                d                5                            ⁡                              (                                  1                  -                                      v                    d                    2                                                  )                                                          60            ⁢                          E              d                        ⁢                          t              d              3                                                          (        2        )            
where Wd [m] designates the width of the air damper, td [m] designates the thickness of the air damper, ld [m] designates the length of the air damper, Ed [Pa] designates the elastic coefficient of the air damper, and xcexdd designates the Poisson""s ratio of the air damper. That is, the acoustic capacitance cd added by the air damper 26 is proportional to the fifth power of the air damper width Wd. In the ink jet recording head shown in FIG. 23, the width of the air damper 26 is equal to the width of the common flow path 4. In order to set the acoustic capacitance cd at a large value, the width of the common flow path 4 has to be set to be large. The total acoustic capacitance cp of the common flow path is a value of the sum of cv and cd.
As described above, in order to secure a large acoustic capacitance in the common flow path 4, it is necessary to set the width of the common flow path 4 to be large. However, in order to secure a large width in the common flow path 4 in the related-art matrix-array head as shown in FIG. 28, it is necessary to set the nozzle pitch Pc to be very large to thereby bring about reduction in the nozzle array density inevitably. That is, in the related-art matrix-array head as shown in FIG. 28, in which common flow paths are formed into straight lines, the nozzle pitch Pc has to be set to satisfy the following expression because of its structural requirements.
Pcxe2x89xa7Wp+dT+2WW1xe2x80x83xe2x80x83(3)
where WP designates the required width of the flow path 23, dT designates the diameter of the communication path 3, and WW1 designates the width of the partition wall between the communication path 3 and the flow path 23. This is because the communication path 3 is formed in the same plane as the flow path 23 in this matrix-array head, and both the communication path 3 and the flow path 23 have to be separated from each other by the partition wall 27.
The communication path 3 is required to have a function to introduce ink into the nozzle 2 in low fluid resistance while stabilizing the direction in which the ink droplet 8 is ejected. To this end, the communication path 3 has to have a certain large diameter. The diameter is about 100-150 xcexcm in a typical ink jet recording head. In addition, the partition wall 27 between the communication path 3 and the flow path 23 needs a certain width to secure a joint to the nozzle plate or the like. For example, when the flow path plate 25 and the nozzle plate 11 are joined to each other by a bonding agent, a failure in bonding is apt to occur when the width of the partition wall 27 is set to be smaller than 100 xcexcm. When there occurs a failure in bonding, a pressure wave leaks between the communication path 3 and the flow path 23. Thus, there arises such a problem that the pressure wave cannot be generated normally. Since each of the communication path diameter dT and the partition wall width WW1 has to be not smaller than a predetermined value, it is difficult to set the nozzle pitch Pc to be small enough to attain a high nozzle array density in related-art matrix-array heads having straight flow paths 23.
On the basis of the finding, an ink jet recording head according to the invention includes: a plurality of ejectors arrayed two-dimensionally, each including a pressure generating chamber, a nozzle communicating with the pressure generating chamber, and a pressure generating portion; and an ink supply system including a common flow path for interconnecting a plurality of the ejectors therewith. The ink jet recording head has a feature in that: ink is charged into the pressure generating chambers through the common flow path; a change of pressure is produced in the ink in the pressure generating chambers by the pressure generating portions corresponding thereto and ink droplets are ejected from the nozzles; the common flow path is disposed to overlap the pressure generating chambers two-dimensionally; and the common flow path has a constricted shape having wide portions and narrow portions.
As for the constricted shape, it is preferable that the common flow path is set to be narrow between the nozzles opposed to each other through the common flow path and wide in any other portion. In the ink jet recording head according to the invention, the common flow path is formed into a constricted shape so that the width of the common flow path is enlarged partially. Accordingly, sufficient acoustic capacitance can be secured in spite of the nozzle pitch Pc set to be smaller than that in the related-art head. In the ink jet recording head according to the invention, the nozzle pitch Pc can be set to be in the following range:
Pcxe2x89xa7W1+WW2xe2x80x83xe2x80x83(4)
where W1 designates the width of the common flow path and WW2 designates the partition wall width between the common flow paths (see FIG. 5). The partition wall width WW2 takes a value substantially equal to the partition wall width WW1. However, pressure wave leakage between the common flow paths causes no great problem on the ejection characteristic. It is therefore possible to set the partition wall width WW2 to be smaller than the partition wall width WW1. That is, in the ink jet recording head according to the invention, the nozzle pitch Pc can be reduced by at least (dT+WW1) in comparison with the related-art matrix-array head. Thus, the nozzle array density can be increased on a large scale.
In addition, the ink jet recording head according to the invention has a feature in that a member forming the nozzles also has a function as an air damper for the common flow path.
With this feature, the air damper for the common flow path can be formed out of a reduced number of members, so that there can be obtained an effect that an ink jet recording head having a high nozzle array density can be produced at low manufacturing cost.
In addition, in the ink jet recording head according to the invention, the member forming the nozzles is made of a resin film.
In this manner, large acoustic capacitance can be secured in the air damper so that required acoustic capacitance can be obtained in the narrower common flow path. Thus, the nozzle array density can be increased further.
In addition, in the ink jet recording head according to the invention, the acoustic capacitance cp of the common flow path is set to satisfy the following conditional expression:
xe2x80x83cp greater than 20cc
In this manner, acoustic crosstalk among the ejectors can be prevented from occurring, so that there can be obtained an effect that an ink jet recording head whose ejecting characteristic is high in uniformity and stability can be produced.
In addition, in the ink jet recording head according to the invention, the acoustic capacitance cp of the common flow path is set to satisfy the following conditional expression:
cp greater than 10cn
In this manner, refill time at the time of concurrent ejection from a plurality of ejectors can be prevented from increasing, so that there can be obtained an effect that the uniformity and the stability in the ejecting characteristic can be improved further.
In addition, in the ink jet recording head according to the invention, flow path resistance of the ink supply system is set so that refill time when ink droplets are ejected continuously from the nozzles is prevented from being longer than an intended ejection period due to a drop in pressure in the common flow path caused by a quasi-stationary ink flow in the ink supply system.
Further, the flow path resistance of the ink supply system is set so that the drop in pressure in the common flow path when ink droplets are ejected continuously from the nozzles is not higher than 800 Pa.
In this manner, an unstable ejection phenomenon appearing when large-diameter ink droplets are ejected concurrently from a plurality of ejectors at a high frequency can be suppressed. Thus, it is possible to produce an ink jet recording head suitable for high speed recording.
In addition, in the ink jet recording head according to the invention, a planar shape of the common flow path corresponding to the constricted shape is formed out of a smooth curve.
In this manner, the flow of ink in the common flow path is made so uniform that bubbles can be prevented from staying in the common flow path. Thus, it is possible to produce an ink jet recording head which is high in reliability.
In addition, in the ink jet recording head according to the invention, the ink supply system includes a plurality of common flow paths; and a second common flow path for interconnecting a plurality of the common flow paths with one another.
In this manner, ink can be supplied efficiently to a large number of ejectors, so that there can be obtained an effect that the size of the head as a whole can be reduced.
In addition, in the ink jet recording head according to the invention, an ink supply port for supplying ink to the second common flow path is provided near the center of the second common flow path.
In this manner, required width of the second common flow path can be reduced, so that there can be obtained an effect that the head size can be reduced.
In addition, in the ink jet recording head according to the invention, a plurality of ink supply ports for supplying ink to the second common flow path are provided in the second common flow path.
In this manner, required width of the second common flow path can be reduced. Thus, it is possible to produce an ink jet recording head in which the head size can be reduced, while the ink jet recording head is hardly affected by clogging of the flow paths with dust or the like.
In addition, in the ink jet recording head according to the invention, a plurality of second common flow paths are provided.
In this manner, the required width of the second common flow paths can be reduced. Thus, there can be obtained effects that the head size can be reduced and bubbles can be prevented from staying in the second common flow paths.
In addition, in the ink jet recording head according to the invention, the common flow paths are disposed substantially in parallel with a main-scanning direction of the ink jet recording head, and the second common flow path is disposed substantially perpendicularly to the main-scanning direction.
In this manner, the total head length in the sub-scanning direction can be set to be short. Thus, there can be obtained an effect that the distance between rollers for conveying a recording medium is set to be so short that the conveyance of the recording medium can be stabilized.
In addition, in the ink jet recording head according to the invention, the common flow paths are disposed substantially perpendicularly to a main-scanning direction of the ink jet recording head; and the second common flow path is disposed substantially in parallel with the main-scanning direction.
In this manner, the total head width in the main-scanning direction can be set to be short, so that it is possible to produce an ink jet recording head more advantageous to high speed recording.
In addition, in the ink jet recording head according to the invention, the plurality of common flow paths are divided into two or more groups, and the respective groups of the common flow paths are connected to the second common flow paths different from one another.
In this manner, the required sectional area of the common flow paths and/or the required sectional area of the second common flow path can be reduced, so that there can be obtained an effect that the nozzle array density can be further increased.
An ink jet recording apparatus according to the invention has a feature in that the ink jet recording apparatus has such an ink jet recording head.
According to such an ink jet recording apparatus, it is possible to produce an ink jet recording apparatus having an extremely high recording speed.